powerpc/mm: Always use STRICT_MM_TYPECHECKS

Testing done by Paul Mackerras has shown that with a modern compiler
there is no negative effect on code generation from enabling
STRICT_MM_TYPECHECKS.

So remove the option, and always use the strict type definitions.

Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>